gitlab_runner: Refactor libvirt-executor

Changes:
- Switch to arch-boxes' base image
- Verify the base image's signature
- Use the new "latest" symlink, instead of parsing the HTML for
  finding the latest arch-boxes image[1]
- Create the base image by using arch-chroot and friends, instead of
  creating a full-blown VM
- Create the VMs from domain XML template instead of virt-clone
- Switch mirror to geo.mirror.pkgbuild.com
- Try to follow "filesystem hierarchy" standards for where to place
  configuration (id_rsa) and "vendor data" (arch-boxes.asc and
  domain_template.xml)
- Misc fixes and cleanups

[1] archlinux/infrastructure!552